Coupling of Mn2O3 with Heteroatom-Doped Reduced Graphene Oxide Aerogels with Improved Electrochemical Performances for Sodium-Ion Batteries

Currently, efforts to address the energy needs of large-scale power applications have expedited the development of sodium–ion (Na–ion) batteries. Transition-metal oxides, including Mn2O3, are promising for low-cost, eco-friendly energy storage/conversion. Due to its high theoretical capacity, Mn2O3 is worth exploring as an anode material for Na-ion batteries; however, its actual application is constrained by low electrical conductivity and capacity fading. Herein, we attempt to overcome the problems related to Mn2O3 with heteroatom-doped reduced graphene oxide (rGO) aerogels synthesised via the hydrothermal method with a subsequent freeze-drying process. The cubic Mn2O3 particles with an average size of 0.5–1.5 µm are distributed to both sides of heteroatom-doped rGO aerogels layers. Results indicate that heteroatom-doped rGO aerogels may serve as an efficient ion transport channel for electrolyte ion transport in Mn2O3. After 100 cycles, the electrodes retained their capacities of 242, 325, and 277 mAh g−1, for Mn2O3/rGO, Mn2O3/nitrogen-rGO, and Mn2O3/nitrogen, sulphur-rGO aerogels, respectively. Doping Mn2O3 with heteroatom-doped rGO aerogels increased its electrical conductivity and buffered volume change during charge/discharge, resulting in high capacity and stable cycling performance. The synergistic effects of heteroatom doping and the three-dimensional porous structure network of rGO aerogels are responsible for their excellent electrochemical performances.


Introduction
Sodium-ion (Na-ion) batteries have good prospects as a replacement for lithium-ion (Li-ion) batteries used in electronic devices due to their low cost, vast quantity of sodium resources, and high energy density batteries, which are comparable to those of Li-ion energy storage devices [1][2][3]. Na-ion energy storage device deployment remains hampered by issues with rate performance, Coulombic efficiency, and cycle stability, among other factors. The main reason for this is that Na + has a larger radius (0.102 nm) than Li + (0.076 nm), resulting in slower reaction kinetics for the sodiation/desodiation of Na + [4]. Ideally, anode materials should have microscopic internal structures that can store Na + and enable a Scheme 1. The schematic representation for the synthesis of Mn2O3/rGO aerogels and Mn2O3/heteroatom-doped rGO aerogels.
The Mn2O3/heteroatom-doped rGO aerogel demonstrates an impressive discharge capacity of up to 100 cycles with remarkable Coulombic efficiencies of up to 99% in comparison with the undoped rGO. The strategy adopted here is useful in enhancing the electrode's capacity and cycling stability. Additionally, the presence of rGO aerogel might improve conductivity and enable quick ion transport inside the electrode. The enhanced specific capacity and cyclability demonstrated by the Mn2O3/rGO and Mn2O3/heteroatomdoped rGO aerogels in our work provide new information for other researchers attempting to forecast the potential for this nanocomposite to be used in Na-ion batteries.

Synthesis of Mn2O3/rGO and Mn2O3/Heteroatom-Doped rGO Aerogels
GO was synthesised according to the modified Hummer method [35]. Mn2O3 was prepared using the hydrothermal method and then thermally decomposed, as previously reported [17]. To synthesise Mn2O3/rGO aerogel, as prepared, GO (90 mg) was dispersed in 18 mL deionised (DI) water, and then, Mn2O3 was added into the above solution. After that, the solution was transferred to a stainless-steel autoclave with a capacity of 125 mL and heated at 180 °C for 12 h. The obtained precipitates (Mn2O3/rGO hydrogel) were then freeze-dried to obtain the Mn2O3/rGO aerogel. A similar procedure was adopted to synthesise the Mn2O3/heteroatom-doped rGO aerogel, where 4 mL NH3 as the N source was added to the suspension and 45 mg L-Cysteine was added as a nitrogen/sulphur source. The collected powders were denoted as Mn2O3/N-rGO and Mn2O3/N,S-rGO aerogels. The Mn 2 O 3 /heteroatom-doped rGO aerogel demonstrates an impressive discharge capacity of up to 100 cycles with remarkable Coulombic efficiencies of up to 99% in comparison with the undoped rGO. The strategy adopted here is useful in enhancing the electrode's capacity and cycling stability. Additionally, the presence of rGO aerogel might improve conductivity and enable quick ion transport inside the electrode. The enhanced specific capacity and cyclability demonstrated by the Mn 2 O 3 /rGO and Mn 2 O 3 /heteroatomdoped rGO aerogels in our work provide new information for other researchers attempting to forecast the potential for this nanocomposite to be used in Na-ion batteries.

Synthesis of Mn 2 O 3 /rGO and Mn 2 O 3 /Heteroatom-Doped rGO Aerogels
GO was synthesised according to the modified Hummer method [35]. Mn 2 O 3 was prepared using the hydrothermal method and then thermally decomposed, as previously reported [17]. To synthesise Mn 2 O 3 /rGO aerogel, as prepared, GO (90 mg) was dispersed in 18 mL deionised (DI) water, and then, Mn 2 O 3 was added into the above solution. After that, the solution was transferred to a stainless-steel autoclave with a capacity of 125 mL and heated at 180 • C for 12 h. The obtained precipitates (Mn 2 O 3 /rGO hydrogel) were then freeze-dried to obtain the Mn 2 O 3 /rGO aerogel. A similar procedure was adopted to synthesise the Mn 2 O 3 /heteroatom-doped rGO aerogel, where 4 mL NH 3 as the N source was added to the suspension and 45 mg L-Cysteine was added as a nitrogen/sulphur source. The collected powders were denoted as Mn 2 O 3 /N-rGO and Mn 2 O 3 /N,S-rGO aerogels.

Physical Characterisation
X-ray diffraction (XRD; Rigaku Miniflex II, Tokyo, Japan) was used to determine the crystal structure. The amount of Mn 2 O 3 in the nanocomposite was quantified using a thermogravimetric analyser (TGA; Perkin Elmer, Waltham, MA, USA) at a heating rate of 10 min −1 in air. A scanning electron microscope (SEM; JSM-6360L; JEOL, Akishima, Tokyo, Japan) and transmission electron microscopy (TEM; TECNAI G2 20 S-TWIN FEI Company, Lincoln, NE, USA) were used to examine the morphology of the samples. Fourier transform infrared (FTIR) measurement was conducted using an IR Tracer-100 from Shimadzu (Kyoto, Japan). Raman spectroscopy (Renishaw, Gloucestershire, UK) was used to collect Raman spectra. X-ray photoelectron spectroscopy (XPS) was used to inspect the chemical states of the elements present in the composite powders using an Axis Ultra DLD XPS (Kratos, Manchester, UK), and the obtained spectra were fitted using CASA software.

Electrochemical Characterisation
The active materials, carbon black and PVDF (weight ratio of 75:20:5), were dissolved in NMP and then coated on a copper foil and left to dry at 100 • C overnight. The fabrication of CR2032 coin-type cells was conducted in an argon-filled glovebox (Unilab, MBRAUN, Germany, H 2 O, O 2 < 0.1 ppm). The active material electrode was sandwiched with sodium metal (Sigma-Aldrich, 99.9%) and separated using a glass fibre (GF/D; Whatman). The electrolyte used was 1 M NaClO 4 in PC with the addition of 5 wt.% FEC. A Neware battery analyser was used to investigate the galvanostatic charge/discharge behaviour, and cyclic voltammetry (CV) was conducted using the CHI 700E between 0.01 and 3.0 V (vs. Na + /Na).

Results and Discussion
The TGA was conducted in the air to estimate the amount of Mn 2 O 3 in the nanocomposites ( Figure 1). The weight loss from ambient until 100 • C was mainly due to the evaporation of the physisorbed water, and then the weight loss up to 400 • C occurred due to the decomposition of the hydroxyl and carboxyl groups [36] present in the rGO. Whereas, for the N-rGO and N,S-rGO aerogels, the weight loss between 150 • C and 600 • C resulted from the decomposition of nitrogen and/or sulphur-containing functional groups, respectively, and was also related to the decarboxylation and elimination of hydroxyl functionalities [37]. The rGO is completely decomposed after 600 • C [38]. Therefore, the amounts of Mn 2 O 3 in Mn 2 O 3 /rGO, Mn 2 O 3 /N-rGO, and Mn 2 O 3 /N,S-rGO aerogels were estimated to be 15, 16, and 19 wt%, respectively.

Physical Characterisation
X-ray diffraction (XRD; Rigaku Miniflex II, Tokyo, Japan) was used to determin crystal structure. The amount of Mn2O3 in the nanocomposite was quantified using a mogravimetric analyser (TGA; Perkin Elmer, Waltham, MA, USA) at a heating rate min −1 in air. A scanning electron microscope (SEM; JSM-6360L; JEOL, Akishima, To Japan) and transmission electron microscopy (TEM; TECNAI G2 20 S-TWIN FEI C pany, Lincoln, NE, USA) were used to examine the morphology of the samples. Fo transform infrared (FTIR) measurement was conducted using an IR Tracer-100 from madzu (Kyoto, Japan). Raman spectroscopy (Renishaw, Gloucestershire, UK) was us collect Raman spectra. X-ray photoelectron spectroscopy (XPS) was used to inspec chemical states of the elements present in the composite powders using an Axis Ultra XPS (Kratos, Manchester, UK), and the obtained spectra were fitted using CASA softw

Electrochemical Characterisation
The active materials, carbon black and PVDF (weight ratio of 75:20:5), were disso in NMP and then coated on a copper foil and left to dry at 100 °C overnight. The fab tion of CR2032 coin-type cells was conducted in an argon-filled glovebox (Un MBRAUN, Germany, H2O, O2 < 0.1 ppm). The active material electrode was sandw with sodium metal (Sigma-Aldrich, 99.9%) and separated using a glass fibre (GF/D; W man). The electrolyte used was 1 M NaClO4 in PC with the addition of 5 wt.% FE Neware battery analyser was used to investigate the galvanostatic charge/discharg haviour, and cyclic voltammetry (CV) was conducted using the CHI 700E between and 3.0 V (vs. Na + /Na).

Results and Discussion
The TGA was conducted in the air to estimate the amount of Mn2O3 in the nano posites ( Figure 1). The weight loss from ambient until 100 °C was mainly due to the e oration of the physisorbed water, and then the weight loss up to 400 °C occurred d the decomposition of the hydroxyl and carboxyl groups [36] present in the rGO. Whe for the N-rGO and N,S-rGO aerogels, the weight loss between 150 °C and 600 °C res from the decomposition of nitrogen and/or sulphur-containing functional groups, re tively, and was also related to the decarboxylation and elimination of hydroxyl func alities [37]. The rGO is completely decomposed after 600 °C [38]. Therefore, the amo of Mn2O3 in Mn2O3/rGO, Mn2O3/N-rGO, and Mn2O3/N,S-rGO aerogels were estimat be 15, 16, and 19 wt%, respectively.   The XRD was used to investigate the structure of the Mn 2 O 3 and its nanocomposites ( Figure 2). No impurity phases were observed, and the Mn 2 O 3 can be indexed to the cubic structure of Mn 2 O 3 (JCPDS no. 11061). The presence of Mn 2 O 3 in the nanocomposites indicates that the Mn 2 O 3 was successfully embedded into the rGO layer structure. The broad diffraction peak appeared at 25 • and was ascribed to the layered and hexagonal structure of the (002) lattice plane of the graphite-like structure [39]. The intensity of the diffraction peaks of the Mn 2 O 3 was decreased in the Mn 2 O 3 -containing samples due to a large amount of rGO aerogel, as confirmed via the TGA.
Nanomaterials 2023, 13, x FOR PEER REVIEW The XRD was used to investigate the structure of the Mn2O3 and its nanocom ( Figure 2). No impurity phases were observed, and the Mn2O3 can be indexed to th structure of Mn2O3 (JCPDS no. 11061). The presence of Mn2O3 in the nanocompos dicates that the Mn2O3 was successfully embedded into the rGO layer structure. Th diffraction peak appeared at 25° and was ascribed to the layered and hexagonal st of the (002) lattice plane of the graphite-like structure [39]. The intensity of the dif peaks of the Mn2O3 was decreased in the Mn2O3-containing samples due to a large of rGO aerogel, as confirmed via the TGA. The morphology of all samples was viewed using SEM ( Figure 3) and TEM 4). Figure 3a-f shows that SEM images of the crumpled and wrinkled structure layers can be observed in all rGO aerogel-containing samples. The porous struc rGO aerogel was interconnected through physical cross-linking to form a net structure with pores smaller than 1 µ m. The porous structure could act as an e channel to shorten the Na-ion diffusion pathway for fast kinetics of Na-ion transp provide a cushion to accommodate the volume changes during the charge/dischar cesses [40]. Figures 3b, d, and f show that the Mn2O3 cubic microstructures of 0.5are distributed on the surface of the rGO layers, which are aggregates of individual This suggests that Mn2O3 cubic structures and rGO aerogel layers assembled eff during hydrothermal. Figure 3g shows that the pristine Mn2O3 possesses a cubic m ogy with a size ranging from 1.1 to 1.9 µ m. Furthermore, the Mn2O3 is anchored The morphology of all samples was viewed using SEM ( Figure 3) and TEM ( Figure 4). Figure 3a-f shows that SEM images of the crumpled and wrinkled structure of rGO layers can be observed in all rGO aerogel-containing samples. The porous structure of rGO aerogel was interconnected through physical cross-linking to form a networked structure with pores smaller than 1 µm. The porous structure could act as an effective channel to shorten the Na-ion diffusion pathway for fast kinetics of Na-ion transport and provide a cushion to accommodate the volume changes during the charge/discharge processes [40]. Figure 3b,d,f show that the Mn 2 O 3 cubic microstructures of 0.5-1.5 µm are distributed on the surface of the rGO layers, which are aggregates of individual Mn 2 O 3 . This suggests that Mn 2 O 3 cubic structures and rGO aerogel layers assembled efficiently during hydrothermal. Figure 3g shows that the pristine Mn 2 O 3 possesses a cubic morphology with a size ranging Nanomaterials 2023, 13, 732 6 of 17 from 1.1 to 1.9 µm. Furthermore, the Mn 2 O 3 is anchored on the surface of the heteroatomdoped rGO aerogel layers and could act as a spacer to prevent the rGO aerogel layers from restacking [41,42].
Nanomaterials 2023, 13, x FOR PEER REVIEW   To support the presence of heteroatoms in the nanocomposites, Figure 5 shows the FTIR spectra of Mn2O3, Mn2O3/rGO, Mn2O3/N-rGO, and Mn2O3/N,S-rGO aerogels. The apparent peaks at 1730 cm −1 (C=O stretching vibration), 1363 cm −1 (O-H deformation), and 1215 cm −1 (C-O stretching vibration) indicate that GO is successfully reduced and the corresponding product is rGO [43,44]. Another FTIR peak at 1440 cm −1 attributed to the C-O stretching deformation and at 1569 cm −1 related to the aromatic skeletal v (C=C) stretching vibration of rGO aerogel is also observed [45]. The existence of S atoms on the N,S-rGO and Mn2O3/N,S-rGO aerogels was established via the appearance of peaks at 1096 cm −1 (C=S) and 2451 cm −1 (S-H stretching vibration) [46]. Furthermore, the C-N stretching vibration located at 1422 cm −1 is an indicator that nitrogen is successfully doped in the nitrogen-containing nanocomposites. The FTIR spectra of Mn2O3 also appeared in the Mn2O3/rGO, Mn2O3/N-rGO, and Mn2O3/N,S-rGO aerogels, in which the Mn-O stretching vibration appeared at 541 and 675 cm −1 [47]. Hence, the FTIR results indicate that there is a Mn-O, C-N, C=S and N-H bonding and thus confirmed that Mn2O3 nanoparticles were incorporated into the rGO aerogel layers. To support the presence of heteroatoms in the nanocomposites, Figure 5 shows the FTIR spectra of the corresponding product is rGO [43,44]. Another FTIR peak at 1440 cm −1 attributed to the C-O stretching deformation and at 1569 cm −1 related to the aromatic skeletal v (C=C) stretching vibration of rGO aerogel is also observed [45]. The existence of S atoms on the N,S-rGO and Mn 2 O 3 /N,S-rGO aerogels was established via the appearance of peaks at 1096 cm −1 (C=S) and 2451 cm −1 (S-H stretching vibration) [46]. Furthermore, the C-N stretching vibration located at 1422 cm −1 is an indicator that nitrogen is successfully doped in the nitrogen-containing nanocomposites. The FTIR spectra of Mn 2 O 3 also appeared in the Mn 2 O 3 /rGO, Mn 2 O 3 /N-rGO, and Mn 2 O 3 /N,S-rGO aerogels, in which the Mn-O stretching vibration appeared at 541 and 675 cm −1 [47]. Hence, the FTIR results indicate that there is a Mn-O, C-N, C=S and N-H bonding and thus confirmed that Mn 2 O 3 nanoparticles were incorporated into the rGO aerogel layers. Raman spectra for all samples are also recorded ( Figure 6). Two typical characteristic peaks of carbon are observed in all samples: the D band at 1340 cm −1 , which reflects the defect and disorder of carbon in graphitic layers, and the G band at 1590 cm −1 , which associates with the characteristics of the E2g mode of sp 2 hybridised carbon [48,49]. The intensity ratio between the D and G bands (ID/IG) can be used to assess surface defects. According to this figure, the ID/IG of rGO aerogel is 1.33. After nitrogen/sulphur doping, the ID/IG of N-rGO and N,S-rGO aerogels were calculated to be 0.95 and 0.98, respectively. After nitrogen and/or sulphur doping, rGO layer defects were established, and the ID/IG ratio was altered via the introduction of cubic Mn2O3 into graphene layers. By contrast, the calculated intensity ratios for Mn2O3/rGO, Mn2O3/N-rGO, and Mn2O3/N,S-rGO aerogels were 1.35, 1.00, and 1.02, respectively. Hence, the band located at ~340 cm −1 could be ascribed to the asymmetric stretch of bridge oxygen species (Mn-O-Mn), in correspondence to that obtained for bulk Mn2O3 particles [50]. Raman spectra for all samples are also recorded ( Figure 6). Two typical characteristic peaks of carbon are observed in all samples: the D band at 1340 cm −1 , which reflects the defect and disorder of carbon in graphitic layers, and the G band at 1590 cm −1 , which associates with the characteristics of the E 2g mode of sp 2 hybridised carbon [48,49]. The intensity ratio between the D and G bands (I D /I G ) can be used to assess surface defects. According to this figure, the I D /I G of rGO aerogel is 1.33. After nitrogen/sulphur doping, the I D /I G of N-rGO and N,S-rGO aerogels were calculated to be 0.95 and 0.98, respectively. After nitrogen and/or sulphur doping, rGO layer defects were established, and the I D /I G ratio was altered via the introduction of cubic Mn 2 O 3 into graphene layers. By contrast, the calculated intensity ratios for Mn 2 O 3 /rGO, Mn 2 O 3 /N-rGO, and Mn 2 O 3 /N,S-rGO aerogels were 1.35, 1.00, and 1.02, respectively. Hence, the band located at~340 cm −1 could be ascribed to the asymmetric stretch of bridge oxygen species (Mn-O-Mn), in correspondence to that obtained for bulk Mn 2 O 3 particles [50].   (Figures 7a, b). Furthermore, the O 1s, C 1s, N 1s, Mn2p3/2, and Mn 2p1/2 peaks are located at 532, 284, 399, 641, and 653 eV and are found in the survey spectrum. Figure 7b exhibits the S 2p peak located at 164 eV. Figure 7c, d shows that the C 1s region can be resolved into three peaks. The peak at 284 eV corresponds to the C-C signal, and 287 eV is related to the C=O, whereas, the peak located at 285.2 eV is related to the C-O and C-N signal for Mn2O3/N-rGO aerogel and C-O, C-N and C-S at 285.3 eV for the Mn2O3/N,S-rGO aerogel. Figure 7e, f shows the O 1s deconvoluted XPS spectra, and the peaks located at 531, 533, and 535 eV corresponded to C-O-Mn, C=O, and surface absorbed oxygen (O-C-O/C-OH), respectively. Moreover, one peak that appeared at 529 eV, as depicted in Figure 7e, might be related to Mn-O. In the N 1s spectra (Figures 7g, h), three fitted peaks can be assigned to pyridinic N (397 eV), pyrrolic N (399 eV), and graphitic N (401 eV) doped in graphene. It has also been reported that the existence of pyridinic-and pyrrolic-type nitrogens can facilitate the transfer of Na-ion and electrons, improving the rate capability of the electrodes [51][52][53]. Additionally, the N concentration in Mn2O3/N-rGO aerogel was 15.54%, whereas, for the Mn2O3/N,S-rGO aerogel, the concentration of N and S is 3.36% and 3.41%, respectively. The highresolution Mn 2p (Figures 7i, j) can be fitted into Mn 2p3/2 (641 eV) and Mn 2p1/2 (653 eV). The observed spin-orbit splitting represented by the difference in binding energy values of these two levels is 12 eV, in line with the previous report [54]. Moreover, the S 2p can be fitted into three components of S p3/2, S p1/2 and SOx centred at 163.7, 164.8 and 168.4 eV (Figure 7k), thus confirming the introduction of S atoms into disordered graphene lattice [55].   (Figure 7a,b). Furthermore, the O 1s, C 1s, N 1s, Mn2p 3/2 , and Mn 2p 1/2 peaks are located at 532, 284, 399, 641, and 653 eV and are found in the survey spectrum. Figure 7b exhibits the S 2p peak located at 164 eV. Figure 7c,d shows that the C 1s region can be resolved into three peaks. The peak at 284 eV corresponds to the C-C signal, and 287 eV is related to the C=O, whereas, the peak located at 285.  (Figure 7g,h), three fitted peaks can be assigned to pyridinic N (397 eV), pyrrolic N (399 eV), and graphitic N (401 eV) doped in graphene. It has also been reported that the existence of pyridinic-and pyrrolic-type nitrogens can facilitate the transfer of Na-ion and electrons, improving the rate capability of the electrodes [51][52][53]. Additionally, the N concentration in Mn 2 O 3 /N-rGO aerogel was 15.54%, whereas, for the Mn 2 O 3 /N,S-rGO aerogel, the concentration of N and S is 3.36% and 3.41%, respectively. The high-resolution Mn 2p (Figure 7i,j) can be fitted into Mn 2p 3/2 (641 eV) and Mn 2p 1/2 (653 eV). The observed spin-orbit splitting represented by the difference in binding energy values of these two levels is 12 eV, in line with the previous report [54]. Moreover, the S 2p can be fitted into three components of S p 3/2 , S p 1/2 and SO x centred at 163.7, 164.8 and 168.4 eV (Figure 7k), thus confirming the introduction of S atoms into disordered graphene lattice [55]. Nanomaterials 2023, 13, x FOR PEER REVIEW 10 of 17  The electrochemical activity of Mn 2 O 3 and their nanocomposites is evaluated using CV ( Figure 8) at a scan rate of 0.1 mV s −1 over 0.01-3.00 V. All samples show similar reduction and oxidation peaks in their CV curves. In the first cycle (Figure 8a-d), the main cathodic peak at 0.8-0.9 V attributes to the reduction of Mn 2 O 3 to Mn−O and the formation of Na 2 O and solid electrolyte interphase (SEI) layers over the active materials, which can be expressed as Equation (1) [17,56].
Mn irreversible conversion reaction (Equation (2)), and the formation of SEI on the electrode's surface.
MnO + 2Na + + 2e − → Mn + Na2O (2) During the second to fifth cycles, the slight shift of the reduction peak at higher potential and the overlapping CV curves indicate the electrochemical reversibility of Naions. Na-ion batteries exhibited wider and weaker CV peaks in comparison to Li-ion batteries since Na-ions have a larger ionic radius than Li-ion and move between graphitic carbon layers more slowly than Li-ion [57,58]. Figure 8d shows that the Mn2O3/N,S-rGO aerogel exhibited an anodic peak at 1.95 V, which may be due to the sulphur incorporated into the N,S-rGO aerogel [59]. The following reaction (Equation (3)) describes the reversible mechanism during the sodiation/desodiation Mn2O3 + 6Na + +6e − ↔ 2Mn + 3Na2O (3)  Figure 9 represents the typical charge and discharge profiles of the samples for several cycles at 0.1 A g −1 . The long discharge plateau observed at the first cycle could be due to the formation of the SEI layer, which leads to the low initial Coulombic efficiency. The subsequent cycle shows no distinct plateau, which is consistent with the CV curves. The broad peak at 0.01-0.4 V may be ascribed to the Na-ion insertion, the MnO to Mn irreversible conversion reaction (Equation (2)), and the formation of SEI on the electrode's surface.
MnO + 2Na + + 2e − → Mn + Na 2 O During the second to fifth cycles, the slight shift of the reduction peak at higher potential and the overlapping CV curves indicate the electrochemical reversibility of Naions. Na-ion batteries exhibited wider and weaker CV peaks in comparison to Li-ion batteries since Na-ions have a larger ionic radius than Li-ion and move between graphitic carbon layers more slowly than Li-ion [57,58]. Figure 8d shows that the Mn 2 O 3 /N,S-rGO aerogel exhibited an anodic peak at 1.95 V, which may be due to the sulphur incorporated into the N,S-rGO aerogel [59]. The following reaction (Equation (3)) describes the reversible mechanism during the sodiation/desodiation Mn 2 O 3 + 6Na + + 6e − ↔ 2Mn + 3Na 2 O (3) Figure 9 represents the typical charge and discharge profiles of the samples for several cycles at 0.1 A g −1 . The long discharge plateau observed at the first cycle could be due to the formation of the SEI layer, which leads to the low initial Coulombic efficiency. The subsequent cycle shows no distinct plateau, which is consistent with the CV curves. Further evaluation of the electrochemical performances of the Mn2O3, Mn2O3/r Mn2O3/N-rGO, and Mn2O3/N,S-rGO aerogel electrodes was conducted, and Figur shows the results. Figure 10a depicts that the Mn2O3/N-rGO aerogel exhibited the hig irreversible discharge capacity of 2507 mAh g −1 , followed by Mn2O3/N,S-rGO aer (1677 mAh g −1 ), Mn2O3/rGO (828 mAh g −1 ), and pristine Mn2O3 (544 mAh g −1 ) at a cur density of 0.1 A g −1 . The Mn2O3/N-rGO aerogel delivers a reversible discharge capaci 594 mAh g −1 during the second cycle and gradually decreases to 325 mAh g −1 after cycles with 87% capacity retention. For Mn2O3/N,S-rGO aerogel, the reversible disch capacity decreased from 448 mAh g −1 (second cycle) to 279 mAh g −1 (100th cycle) wit 83% retention rate. These results indicate that the heteroatom doping on the rGO aer layers could tailor the physicochemical properties of the rGO, which in turn enhanced discharge capacity and cyclability of the Mn2O3. The advantages of anchoring the M on the rGO layers can also be seen in the electrochemical properties of Mn2O3/rGO aer versus pristine Mn2O3. In the second cycle, the Mn2O3/rGO aerogel exhibits a rever discharge capacity of 305 mAh g −1 and gradually reaches 242 mAh g −1 after 100 cycles a capacity retention of 71%. Whereas, for pristine Mn2O3, the reversible discharge capa at the second cycle was 294 mAh g −1 and decreased to 157 mAh g −1 after 100 cycles 71% capacity retention. During the initial few cycles, the porous structure of rGO aer promoted the formation of excessive SEI layers, resulting in a lower initial Coulombi ficiency [60]. After a few cycles, the Coulombic efficiencies for all electrodes reached due to the relatively stable SEI that formed during the cycling. Additionally, the Mn2O rGO aerogel (Figure 10b) has better rate capability compared with other electrodes w discharge capacities of 260, 222, 201, 177, 232, and 272 mAh g −1 at current densities of 0.4, 0.6, 0.8, 1.0, and returning 0.2 A g −1 , respectively. Interestingly, other electrodes show good rate capability performance when executed at different current densities with slightly lower discharge capacity values compared with Mn2O3/N-rGO aerogel. , and pristine Mn 2 O 3 (544 mAh g −1 ) at a current density of 0.1 A g −1 . The Mn 2 O 3 /N-rGO aerogel delivers a reversible discharge capacity of 594 mAh g −1 during the second cycle and gradually decreases to 325 mAh g −1 after 100 cycles with 87% capacity retention. For Mn 2 O 3 /N,S-rGO aerogel, the reversible discharge capacity decreased from 448 mAh g −1 (second cycle) to 279 mAh g −1 (100th cycle) with an 83% retention rate. These results indicate that the heteroatom doping on the rGO aerogel layers could tailor the physicochemical properties of the rGO, which in turn enhanced the discharge capacity and cyclability of the Mn 2 O 3 . The advantages of anchoring the Mn 2 O 3 on the rGO layers can also be seen in the electrochemical properties of Mn 2 O 3 /rGO aerogel versus pristine Mn 2 O 3 . In the second cycle, the Mn 2 O 3 /rGO aerogel exhibits a reversible discharge capacity of 305 mAh g −1 and gradually reaches 242 mAh g −1 after 100 cycles with a capacity retention of 71%. Whereas, for pristine Mn 2 O 3 , the reversible discharge capacity at the second cycle was 294 mAh g −1 and decreased to 157 mAh g −1 after 100 cycles with 71% capacity retention. During the initial few cycles, the porous structure of rGO aerogel promoted the formation of excessive SEI layers, resulting in a lower initial Coulombic efficiency [60]. After a few cycles, the Coulombic efficiencies for all electrodes reached 90% due to the relatively stable SEI that formed during the cycling. Additionally, the Mn 2 O 3 /N-rGO aerogel (Figure 10b) has better rate capability compared with other electrodes with discharge capacities of 260, 222, 201, 177, 232, and 272 mAh g −1 at current densities of 0.2, 0.4, 0.6, 0.8, 1.0, and returning 0.2 A g −1 , respectively. Interestingly, other electrodes also show good rate capability performance when executed at different current densities, but with slightly lower discharge capacity values compared with Mn 2 O 3 /N-rGO aerogel. The overall electrochemical performances of Mn2O3/heteroatom-doped rGO aerogels reported here are superior to those reported previously (Table 1). It is believed the pronounced electrochemical performances of Mn2O3/heteroatom-doped rGO aerogels can be attributed to the networked structure of the nanocomposites, which have a variety of favourable properties. First, the porous networking of Mn2O3 and rGO aerogel may shorten the diffusion path for Na-ions and facilitate the electrolyte's penetration into the inner surface of the active materials. The interconnected rGO networks in this structure offer an additional active site for Na-ion storage and also offer a large area for reversible reactions [42,61,62]. Second, the 3D rGO aerogel networks improved the mechanical integrity of the electrode by accommodating the strain and stress during the sodiation/desodiation processes. Consequently, structure destruction of the active materials can be avoided while maintaining the cyclability of the electrode. The aggregation of Mn2O3 could also be inhibited [63,64]. Third, the electron transfer ability of the Mn2O3 was enhanced in combination with the heteroatom-doped rGO aerogel. The electrical conductivity of the active materials is greatly improved by decreasing the charge-transfer resistance, which in turn facilitates the strong electronic interconnection of the active materials to ensure rapid charge transfers within the electrode [65]. Lastly, doping nitrogen and co-doping nitrogen/sulphur improve the physicochemical characteristics of the rGO [28]. The addition of heteroatoms to rGO aerogels may improve their electrical conductivity and electrochemical activity by fostering charge transfer between adjacent carbon atoms [45,66,67]. The strategies of doping heteroatoms into rGO aerogels could be ideal platforms to tackle the The overall electrochemical performances of Mn 2 O 3 /heteroatom-doped rGO aerogels reported here are superior to those reported previously (Table 1). It is believed the pronounced electrochemical performances of Mn 2 O 3 /heteroatom-doped rGO aerogels can be attributed to the networked structure of the nanocomposites, which have a variety of favourable properties. First, the porous networking of Mn 2 O 3 and rGO aerogel may shorten the diffusion path for Na-ions and facilitate the electrolyte's penetration into the inner surface of the active materials. The interconnected rGO networks in this structure offer an additional active site for Na-ion storage and also offer a large area for reversible reactions [42,61,62]. Second, the 3D rGO aerogel networks improved the mechanical integrity of the electrode by accommodating the strain and stress during the sodiation/desodiation processes. Consequently, structure destruction of the active materials can be avoided while maintaining the cyclability of the electrode. The aggregation of Mn 2 O 3 could also be inhibited [63,64]. Third, the electron transfer ability of the Mn 2 O 3 was enhanced in combination with the heteroatom-doped rGO aerogel. The electrical conductivity of the active materials is greatly improved by decreasing the charge-transfer resistance, which in turn facilitates the strong electronic interconnection of the active materials to ensure rapid charge transfers within the electrode [65]. Lastly, doping nitrogen and co-doping nitrogen/sulphur improve the physicochemical characteristics of the rGO [28]. The addition of heteroatoms to rGO aerogels may improve their electrical conductivity and electrochemical activity by fostering charge transfer between adjacent carbon atoms [45,66,67]. The strategies of doping heteroatoms into rGO aerogels could be ideal platforms to tackle the problems related to the use of transition-metal oxide as anode materials for Na-ion batteries, especially electrode pulverisation during cycling. Several challenges related to Mn 2 O 3 have to be overcome, and modifications and development of the Mn 2 O 3 with nanocomposites are necessary to achieve the desired electrochemical properties for Na storage for practical applications.

Conclusions
In summary, the integration of Mn 2 O 3 with the 3D framework of heteroatom-doped rGO aerogel was demonstrated to be an effective strategy to mitigate problems such as volume changes during the charge/discharge process, mechanical degradation, and capacity fading. The cubic Mn 2 O 3 particles (with an average size of 0.5-1.5 µm) obtained via thermal decomposition of MnCO 3 are anchored in porous and crumpled rGO sheets of micrometre dimension. This architecture ensures closed contact between the electrolyte and the active materials that enable efficient electron transport from each active Mn 2 O 3 cubic particle to the current collector via the rGO aerogel. The aerogel structure produced heteroatomdoped rGO layers with strong interconnections, and the Mn 2 O 3 particles distributed on the rGO layers prevented the restacking of the rGO layers. A strong synergistic effect between Mn 2 O 3 and heteroatom-doped rGO in the nanocomposite becomes more apparent with cycling and contributes to the excellent cycling performance of the electrode. After 100 cycles, the capacities of the Mn 2 O 3 /rGO, Mn 2 O 3 /N-rGO, and Mn 2 O 3 /N,S-rGO aerogels were retained at 242, 325, and 277 mAh g −1 , respectively. In comparison to Mn 2 O 3 /rGO, the Nand N,S-doped rGO aerogels with Mn 2 O 3 displayed higher specific capacities, excellent cyclability, and rate capability. Thus, Mn 2 O 3 anchored on the heteroatom-doped rGO aerogel could effectively improve the Na storage capacity of Mn 2 O 3 and provide a useful strategy for producing high-yield anode materials for practical Na-ion batteries.

Conflicts of Interest:
The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.